The present invention relates to vehicle detectors which detect the passage or presence of a vehicle over a defined area of a road way. In particular, the present invention relates to a vehicle detector with compensation for periodic noise such as noise produced by power lines near an inductive sensor.
Inductive sensors are used for a wide variety of detection systems. For example, inductive sensors are used in systems which detect the presence of conductive or ferromagnetic articles within a specified area. Vehicle detectors are a common type of detection system in which inductive sensors are used.
Vehicle detectors are used in traffic control systems to provide input data required by a controller to control signal lights. Vehicle detectors are connected to one or more inductive sensors and operate on the principle of an inductance change caused by the movement of a vehicle in the vicinity of the inductive sensor. The inductive sensor can take a number of different forms, but commonly is a wire loop which is buried in the roadway and which acts as an inductor.
The vehicle detector generally includes circuitry which operates in conjunction with the inductive sensor to measure changes in inductance and to provide output signals as a function of those inductance changes. The vehicle detector includes an oscillator circuit which produces an oscillator output signal having a frequency which is dependent on sensor inductance. The sensor inductance is in turn dependent on whether the inductive sensor is loaded by the presence of a vehicle. The sensor is driven as a part of a resonant circuit of the oscillator. The vehicle detector measures changes in inductance of the inductive sensor by monitoring the frequency of the oscillator output signal.
Examples of vehicle detectors are shown, for example, in U.S. Pat. No. 3,943,339 (Koerner et al.) and in U.S. Pat. No. 3,989,932 (Koerner).
If the inductive sensor is located near electric power distribution lines, magnetic flux from the power lines can introduce a periodic fluctuation in the frequency of the oscillator signal which constitutes noise. This fluctuation, which is at the power main frequency (for example 60 Hz) manifests itself as a variation in the value of the measured frequency when no other stimulus is applied to the vehicle detector. If this condition occurs, and depending on the phase of the mains line at which the measurement is taken, the variation may be large enough to cause an apparent reduction in sensitivity or the vehicle detector may continuously register the presence of a vehicle, even when a vehicle is not present.